In the case of refrigerated semi-trailers, truck bodies or other vehicles or cargo containers which are used to maintain a cargo at a desired temperature, the proper circulation of air above and below the cargo is desirable. Refrigeration in particular is used to prevent damage or deterioration of perishable goods, to transport frozen foods or the like. Alternatively in certain cargo applications it may be desirable to maintain a cargo at a temperature above ambient temperature such as to prevent a bottled liquid cargo from freezing during transport in the winter or similar applications.
To promote air circulation below the cargo, various configurations have been used to accomplish the two necessary functions of holding the cargo in a raised position relative to the floor of the trailer and to provide ducts through which conditioned air is permitted to circulate throughout the length of the trailer. In the environment in which these structures are typically used, a number of additional considerations come into play. The structures require the ability to withstand structural loads in excess of the simple weight of the cargo in that transportation may cause various bumps and shifts in the load, loading and unloading may result in dropping of the cargo and typically the cargo is loaded through the use of fork-lifts which drive into the trailer itself contributing loads from the weight of fork-lift plus the cargo distributed on the relatively small surface area in contact with the fork-lift wheels.
Various solutions to these problems have been attempted and patented including various combinations of elements. One element is typically called a "T" section because the upstanding elements form a section in a "T" shape. Another section is known as a duct section which typically uses uniform width corrugations with angled walls and flat top elements for supporting a load and flat bottom elements carried on the sub-pan of the trailer.
As has been noted in the prior art, the "T" sections provide less restriction to the longitudinal air flow but are more difficult to clean and not as strong as the duct construction. An example of the "T" construction is shown in U.S. Pat. No. 3,368,315 which focused on the interlocking edges of adjacent extruded panels. The duct configuration provides greater strength than the "T" and easier cleaning but less than half the area is available for air circulation. In the duct configuration only the area of the "U" section is open to the cargo compartment while the inverted "U" is effectively closed through its resting on the stringers comprising a portion of the sub-pan of the trailer.
Another alternative to use a combination of "T" section floor board extrusions and duct section extrusions using common interlocking edges. In areas of high load, such as the expected wheel base of fork-lift trucks, a stronger angled wall duct section floor board panel could be utilized, while in areas where lesser load was expected, such as the center of the trailer, the "T" section floor board panels could be used. Because of their interlocking nature, the combination of different section panels or "boards" could thus result in placement of a "T" panel immediately adjacent a duct panel.
U.S. Pat. No. 4,631,891 issued to Donavich provides for the placement of "T" section elements next to angled wall duct elements in the same floor board extrusion. U.S. Pat. No. 3,128,851 issued to Deridder provides another alternative whereby the width of the inverted "U" section, angled-wall corrugation has been reduced to provide a slight increase in the airflow.